The present invention relates to an electroplating bath used for forming a tin-silver alloy plated film as a substitute for a tin-lead alloy solder, and a tin-silver alloy electroplating process using the bath.
In the case of soldering electronic parts, printed boards, and the like, tin plating or tin-lead alloy plating has been mainly performed. However, tin plating has a problem in generation of whiskers and tin-lead plating has a problem that lead contained therein recently becomes a focus as a cause of environmental pollution. As a result, it is expected to develop a tin alloy solder material not containing lead.
To be more specific, in recent years, from the viewpoint of regulation of utilization of lead, it is required to use a tin-silver alloy as a substitute for the conventional tin-lead alloy solder for soldering various parts constituting electronic equipment and the like, such as chip parts, hoop members, lead frames, semiconductor packages, bumps and printed boards, and to meet such a requirement, it is expected to develop an electroplating bath for forming a tin-silver alloy plated film.
As the tin-silver alloy electroplating bath, there is known a bath containing tin alkanesulfonate, silver alkanesulfonate, alkanesulfonic acid, and a nonionic surface active agent, from Japanese Patent Laid-open No. Hei 8-13185.
The conventional tin-silver alloy plating bath, however, tends to produce, during plating work, acicular, dendrite, whisker-like, granular, or powdery deposits in the order of several .mu.m to several mm and/or burnt deposits on an end portion or edge portion or over the entire surface of a workpiece to be plated, resulting in failures in terms of external appearance of the plated surface, film, thickness, alloy composition, short-circuit between adjacent leads, and the like.
The simple bath of this type also has another problem that silver is liable to be deposited in preference to tin, and also silver tends to be deposited by substitution on a tin electrode or tin-silver alloy electrode and the plated film. This obstructs smooth plating work and degrades adhesiveness of the plated film.
A neutral or weak alkaline tin-silver alloy electroplating bath containing stannous chloride, silver iodide, potassium iodide and potassium pyrophosphate, which is different from the above-described strong acidic bath, is disclosed in Abstract of the 93th Meeting of Japan Society of Surface Technology, pp. 195. However, such an electroplating bath, when used for electroplating a printed board, may erode and peel an organic resist film.